A challenge in developing optoelectronic semiconductor devices, such as light emitting diodes (LEDs), laser diodes (LDs), and photodiodes, is the development of an ohmic contact to n-type or p-type semiconductor layers, which has a low specific resistance while also having a high reflectivity or transparency. In general, most ohmic contacts to semiconductor layers are partially rectifying Schottky contacts, however, for the cases where the nonlinear contact behavior can be ignored, such contacts are referred as ohmic. For example, the challenge to manufacture a low resistance ohmic contact to n-type material is particularly important for deep ultraviolet LEDs made from group III-nitride materials, such as Aluminum Gallium Nitride (AlGaN) or Aluminum Gallium Indium Nitride (AlGaInN), which include a high molar fraction of aluminum. Similarly, the challenge for manufacturing quality contacts to p-type nitride semiconductors is important for all nitride-based LEDs, since a relatively low p-doping (e.g., less than 1×1018 cm−3) in p-type GaN makes the formation of such p-type ohmic contacts difficult. Developing a good ohmic contact is particularly challenging for semiconductor layers containing high levels of aluminum. In particular, there are no metals that can match a high work function of p-doped AlGaN alloys. The resulting difference between semiconductor and metal work functions results in formation of a Schottky barrier at the metal semiconductor junction.
Various approaches have been employed to improve contact resistance for both n-type and p-type contacts. One approach, which can produce a good ohmic contact to a semiconductor layer, uses an annealing process. For example, titanium (aluminum gallium) nitride (Ti3(AlGa)N) is frequently used as an ohmic contact to n-doped nitride semiconductor layers. In this case, the titanium nitride (TiN) layer creates N vacancies in the underlying aluminum gallium nitride/gallium nitride (AlGaN/GaN) structure, which effectively dopes the material. Frequently, gold (Au) also is added to prevent diffusion and oxidation of the TiN.
Another approach includes etching semiconductor layers and planting the ohmic contact into the etched cavity. For example, in one approach, recessed ohmic contacts are disclosed where a semiconductor device is formed by engineering a channel-forming layer grown on a semiconductor substrate with subsequent deposition of a Schottky layer. In this approach, the two dimensional electron gas (2DEG) is established at an interface between the Schottky layer and the channel-forming layer. Furthermore, in this approach, a gate electrode is formed on the Schottky layer via a cap layer and a recess-structured ohmic electrode is in ohmic contact with the 2DEG layer.
A similar technique has been used for Metal-Insulator-Semiconductor (MIS) high electron mobility transistors (HEMT). In this case, an insulating two nanometer thick AlN layer is removed and source and drain contacts are recessed. Contrary to the previous approach, the source and drain contacts are not recessed all the way to the 2DEG layer.
Recessed source and drain contacts also have been investigated in the context of transistor devices. Results have shown that a recessed source/drain structure can provide an ohmic contact with a much lower source/drain resistance than a conventional elevated source/drain contact. Furthermore, the recessed source/drain contact can reduce parasitic gate to source/drain capacitance over the conventional approach. A drawback of the recessed source and drain contacts is a presence of a short channel effect, which can deteriorate the device performance.
A recessed ohmic contact is useful as a way to access the 2DEG. A 2DEG is typically utilized in a HEMT, where the current path is formed at an interface between two types of semiconductor film having different band gaps. In order to support the 2DEG, the semiconductor layers typically comprise a channel-forming layer formed on a substrate and another layer forming a heterojunction with the channel-forming layer. For example, a GaN film can be used as the channel-forming layer, and an AlGaN film can be used as the layer forming the heterojunction with the channel-forming layer.
A recessed ohmic contact also is beneficial in cases when semiconductor layers do not support 2DEG, such as an ohmic contact formed for a light emitting device (LED). In this case, the recessed ohmic contact allows for a larger contact-to-semiconductor junction area, and as a result, a lower contact resistance.
Approaches for forming ohmic contacts are very different for n-type and p-type contacts. For n-type contacts to n-type GaN, for example, the ohmic contacts are formed using a metal work function that is smaller than that of the n-type GaN based semiconductor. A frequently used metal is Ti, which has a work function, ϕm=4.33 eV. For Ti-based contacts to n-type GaN, which has a carrier concentration of 5 to 7×1018 cm−3, low contact resistances ranging from 10−5 to 10−8 Ωcm2 have been obtained.
Making a p-type contact to p-type GaN, for example, is much more difficult. In particular, it is difficult to grow well doped p-type GaN with a carrier concentration of more than 1018 cm−3 due to a high activation energy of acceptors. Additionally, it is difficult to find metals with a work function that corresponds to p-type GaN. Metals with a large work function, such as Ni, are typically used to form ohmic p-type contacts. The details of annealing are an important factor for contact performance. Various annealing approaches have been proposed, including annealing in air or oxygen to improve contact performance. Other approaches to improve the performance of an ohmic contact include various methods of treating a semiconductor surface. The possible methods include plasma and laser treatment. In addition, use of superlattices, strained semiconductor layers, and spontaneous polarization have been employed to achieve a high hole concentration and result in a low contact resistivity.
In addition to improving the electrical properties of ohmic contacts, it is desirable to improve the optical properties of these contacts for more efficient operation of optoelectronic devices, such as LEDs, LDs, photodiodes, and the like. For example, for efficient operation of an LED device, the generated light needs to exit the device without significant absorption. Semiconductor LED devices, and particularly group III nitride-based semiconductor LED devices, are composed of semiconductor layers having a high refractive index. Such devices effectively trap the generated light by means of total internal reflection (TIR). Various approaches to improve extraction efficiency have been proposed. These approaches include surface patterning, surface roughening and LED die/LED substrate shaping. Each of these approaches relies on the ohmic contact being partially transparent or reflective, since most of the light absorption occurs at these contacts.
Similar to the various approaches proposed to improve electrical properties of ohmic contacts, various approaches have been proposed to improve the transparency or reflectivity of ohmic contacts. Semi-transparent metal contacts based on Ni/Au have been used to improve light extraction. Indium Tin Oxide (ITO) has been used as a transparent electrode for visible LEDs. Furthermore, strained p-type In0.1Ga0.9N and highly doped InGaN layers have been used in conjunction with ITO to improve its electrical properties.
To improve the reflectivity of an ohmic contact, one approach uses a multi-layered contact containing aluminum and rhodium as a highly reflective ohmic contact. A high quality alloy contact, such as a Ni/Ag/Pt ohmic contact, was used as a reflective contact to a p-type GaN semiconductor layer. In this case, after annealing at 500° C. in O2 ambient for three minutes, a specific contact resistance as low as 2.6×10−5 Ω·cm2 and an optical reflectivity of 82% at 460 nanometers were obtained.
Other approaches utilize distributed Bragg reflectors (DBRs) and omnidirectional reflectors to improve the reflectivity of ohmic contacts. DBRs have a high reflectivity, which strongly depends on both the incidence angle and the polarization of the incident light. As a result, DBRs become transparent for oblique angles of incidence. On the other hand, omnidirectional reflectors are designed to reflect light at various incident angles.
Another approach uses a mesh electrode to improve the extraction efficiency of GaN-based LEDs. A mesh electrode based on rhodium was manufactured for a p-type GaN contact instead of a typical Ni/Au electrode to reduce the optical absorption by the p-type contact electrode. The external quantum efficiency was estimated to be 35.5%. Similarly, striped contact electrodes have been proposed.
Other approaches roughen some of the contact surfaces to improve light extraction efficiency. For example, one approached investigated vertical-structured light emitting diodes (VLEDs) with a GaOx film atop an n-type GaN layer roughened via KrF laser irradiation and a TiO2/SiO2 DBR.